It looks like it will be Prince Caspian, which would follow nicely with the Lion, the Witch and the Wardrobe. It makes the most sense for those who haven't read the books, although it doesn't follow chronologically from the series. In real world time, the four Pevensie kids are only a little older so they can easily use the same four kids to play the roles
http://entertainment.news.com.au/story/0,10221,18026314-10431,00.html